1. Field of the Invention
The present invention relates to a cryogenic system for cooling a superconducting magnet or the like.
2. Description of the Related Art
It is well known that a superconducting magnet apparatus must be kept at cryogenic temperatures to maintain the superconductivity. Examples of cooling means include a method of immersing the superconducting magnet in a cryogen, such as liquid helium, and a method for cooling the superconducting magnet directly with a cryocooler without using a cryogen.
FIG. 13 shows a structure of a conventional cryogenic system. This is magnetic resonance imaging (MRI), which is known as a medical apparatus. FIG. 13 is a longitudinal section of the cryogenic system housing a superconducting solenoid magnet having a horizontal central axis.
This cryogenic system includes a cryocooler unit 51, a vacuum chamber 52, a superconducting magnet 53, a liquid helium bath 55, and a radiation-shield 56. The superconducting magnet 53 is placed in the vacuum chamber 52 and generates a large magnetic field. The liquid helium bath 55 houses the superconducting magnet 53 and contains liquid helium 54 for cooling the superconducting magnet 53. The radiation-shield 56 is provided between the vacuum chamber 52 and the liquid helium bath 55 to shield radiation from the vacuum chamber 52 to the liquid helium bath 55.
The cryocooler unit 51 has two stages; a first cooling stage 57 is thermally coupled to the radiation-shield 56 and a second cooling stage 58 is thermally coupled to a recondenser 59, each stage being cooled at each predetermined temperature. A sleeve 60 housing the cryocooler unit 51 and the liquid helium bath 55 communicate with each other. Spaces over the liquid helium bath 55 and within the cryocooler sleeve 60 are filled with helium having a saturated vapor pressure at an operating temperature of the superconducting magnet 53.
In such a helium recondensing cryogenic system, the cryocooler unit 51 that has a cooling capacity at as low as 4.2 K at the second cooling stage 58 is adopted.
Thus, the recondenser 59 has a surface temperature lower than the temperature of liquid helium. When a gaseous phase of the liquid helium 54 comes into contact with the recondenser 59, it can be recondensed into liquid helium. Thus, in the system shown in FIG. 13, a user does not need to refill liquid helium 54 as long as the cryocooler unit 51 operates, and can use the cryogenic system without monitoring a cryogenic coolant.
However, the operation of the cryocooler unit 51 shown in FIG. 13 is interrupted by periodical replacement of internal components or maintenance of a compressor unit, which supplies compressed gas to the cryocooler unit 51. During such a maintenance, an increased amount of heat is transferred to the liquid helium bath 55, promoting the evaporation of liquid helium 54. Furthermore, evaporated helium is not recondensed by the cryocooler unit 51 during the maintenance and is entirely released from the system.
Thus, liquid helium 54 gradually decreases because of repeated maintenance and must be refilled. This increases the operation cost of the system. In the system shown in FIG. 13, therefore, the maintenance time should be as short as possible to reduce the flow of heat into the liquid helium bath 55.
It should be noted that although the shutdown of the compressor does not take many hours, the maintenance of the cryocooler unit 51 takes considerable time. That is, procedures of removing the cryocooler unit 51 from the cryocooler sleeve 60; putting a new cryocooler unit 51 into the cryocooler sleeve 60; starting the new cryocooler unit 51; and waiting until a steady state is reached cannot be omitted.
Thus, in addition to the prompt replacement of the cryocooler unit 51, the reduction of the time to reach the steady state is required to be shorten. Thus, the maintenance cannot be shortened to a few hours.
The second problem in the maintenance of the cryogenic system relates to thermal resistance between the cryocooler unit 51 and an element to be cooled.
In the structure shown in FIG. 13, to get the best performance from the cryocooler unit 51, a thermal contact between the first cooling stage 57 and the radiation-shield 56, and a thermal contact between the second cooling stage 58 and the recondenser 59 must be reproducibly maintained in good condition. For example, a poor thermal contact between the first cooling stage 57 and the radiation-shield 56 results in a large difference in temperature at the interface (thermal contact resistance) when unit heat passes through the contact. This increases the temperature of the radiation-shield 56 and therefore increases the flow of heat into the liquid helium bath 55. At worst, the flow of heat into the liquid helium bath 55 exceeds the capacity of the recondenser 59; that is, not all the helium evaporated from the liquid helium 54 can be recondensed even when the cryocooler unit 51 is in operation. Thus, cryogen (liquid helium) must be added periodically. This considerably impairs the operationability of the cryogenic system.
A poor thermal joint between the second cooling stage 58 and the recondenser 59 is more serious. In the cryocooler unit 51 shown in FIG. 13, since the second cooling stage 58 has a much smaller cooling capacity than the first cooling stage 57, even a small difference in temperature at the second cooling stage 58 largely affects the recondensation. Even when both thermal joints have the same temperature difference of 1 K, for example, this is obvious when the ratio of the temperature difference to the operating temperatures of the first cooling stage 57 and the second cooling stage 58 are considered.
Thus, when the temperature difference between the second cooling stage 58 and the recondenser 59 is large, the temperature of the recondenser 59 does not decrease sufficiently. At worst, cooling fins become higher in temperature than liquid helium 54 and cannot recondense helium gas. This also leads to the refilling of cryogen.
Increased thermal resistance between the cryocooler unit and an element to be cooled partly results from contamination in the cryocooler sleeve 60.
When the cryocooler unit 51 is removed from the cryocooler sleeve 60, the temperature inside the cryocooler sleeve 60 is lower than the ambient temperature. In general, the temperature is 30 to 60 K at the radiation-shield 56, and about 3 to 5 K at the bottom of the cryocooler sleeve 60. Thus, when perfect measures are not taken to prevent the outside air from entering the cryocooler sleeve 60, the air in the amount corresponding to the cryocooler unit 51 enters from the outside of the vacuum chamber 52. This causes deposition of water vapor and air within the cryocooler sleeve 60. As a result, this decreases the contact area at the interface between the first cooling stage 57 and the radiation-shield 56, increases the thermal resistance, and may cause the same problem as described above.
In the past, many attempts to overcome these two big problems, that is, shortening of the maintenance and reduction of the thermal resistance between the cryocooler unit and the element to be cooled were made.
For example, one described in U.S. Pat. No. 5,918,470 is already known.
This prior art utilizes a hermetic liquid helium container, which eliminates the addition of cryogen. Furthermore, a cooling stage of a cryocooler unit and a recondenser of the liquid helium container (bath) are spaced at a predetermined interval. A thermal joint of an indium gasket is placed in the gap to reduce thermal resistance between the cryocooler unit and an element to be cooled.
However, in the technique described in U.S. Pat. No. 5,918,470, to make effective use of the gasket to decrease the thermal resistance, very high pressure must be applied to pinch the gasket. This may damage the cryocooler unit even with a soft metal gasket like indium.
Since the thermal contact resistance between two materials at cryogenic temperatures decreases as the contact pressure increases, the two materials should be pressed against each other at the highest possible pressure to improve heat transfer at the contact surface of the thermal joint. However, in general, the portion of the cryocooler unit between the first cooling stage and the second cooling stage is often made of very thin material to reduce the heat flow. Thus, the cryocooler unit may be damaged by a large mechanical stress.
On the contrary, an insufficient mechanical stress results in a large thermal resistance. Thus, it is very difficult to adjust the mounting position of the cryocooler unit so that the mechanical stress would not be too large and not too small.
Furthermore, when the cryocooler unit is removed for maintenance, in the cryogenic system with the indium gasket, heat may flow into the liquid helium container from the gasket. Thus, it may take many hours to resume the operation.
In addition, replacement of the gasket after removing the cryocooler unit takes additional time for maintenance.
In an attempt to overcome the problem described above, U.S. Pat. No. 6,164,077 proposed a heat transfer mechanism in which a liquid at a cryogenic temperature is introduced into a thermal joint between a second cooling stage and an element to be cooled, evaporates on the element to be cooled, and recondenses at the second cooling stage.
The prior art described in U.S. Pat. No. 6,164,077 has many advantages. Since the thermal joint is achieved by the evaporation and recondensation of a liquid coolant, a cryocooler unit can be easily removed and mounted. Moreover, since no stress is applied to the cooling stage, the precision of mounting position of the cryocooler unit is not an important issue.
However, for a superconducting magnet made of a superconducting metal wire as in this prior art, the element to be cooled must be maintained at the liquid helium temperature or lower. Thus, a possible thermal joint medium (cryogen) is only liquid helium. Furthermore, two mechanisms, that is, evaporation and recondensation of the cryogen are required in series between the element and the cryocooler unit. This results in a larger difference in temperature and therefore lower efficiency than the thermal transfer via a solid.
Furthermore, both techniques described in U.S. Pat. No. 5,918,470 and U.S. Pat. No. 6,164,077 cannot solve the problem of contamination in the cryocooler sleeve.